Pipetting of liquids is an important function of automatized analysis of samples examined for the purposes of medical diagnosis. Mastering of the pipetting operations is a basic condition for performing analysis which are correct, fast, cheap and ecological. There is a need for a pipetting apparatus which is able to pipette with the required accuracy liquid volumes in the nanoliter and microliter range.
Historically, dispensing of very small liquid volumes requires contact of the pipetting needle with a solid surface or with another liquid to which the dispensed volume is added. This is so because the adhesion forces which retain the small volume to be dispensed to the pipetting needle are larger than the weight of that small liquid volume. This weight alone is thus not sufficient for releasing a drop attached by adhesion forces to the tip of a pipetting needle. In prior art automatic pipetting apparatus of analyzers used for medical diagnosis a drop of a liquid to be dispensed is therefore brought into contact with and thereby delivered into another liquid, which can be a sample or a reagent. In order to avoid erroneous analysis results, it is necessary to clean the pipetting needle after each such contact with liquid in a container and this requires a lot of time.
According to prior art delivery of a liquid to be dispensed can only be achieved by contact of the tip of the pipetting needle with a liquid contained in a container which receives the dispensed liquid. In some applications it is however desirable to dispense a liquid without any contact between the tip of the pipetting needle and a liquid contained in a container that receives the dispensed liquid, since in this case cleaning of the needle would not be necessary after each dispensing operation. This is the case for example, when aliquots of a liquid sample are to be distributed to liquids contained in a plurality of containers. In this case the time for distributing the aliquots to the plurality of containers would be considerably reduced, because it would not be necessary to clean the pipetting needle after dispensing each aliquot.
FIG. 1 shows a prior art dispensing of very small aliquots of a liquid to a plurality of different vessels. As shown in FIG. 1, a pipetting needle 11 is used for taking a sample of liquid contained in a vessel 12 and for successively dispensing aliquots of that sample to different vessels 13 and 14. For each such dispensing the tip needle 11 has to contact a liquid contained in the vessel 13, 14 which receives the aliquot. For the reasons mentioned above the needle 11 has to be cleaned after each such dispensing, before dispensing an aliquot in a different vessel. FIG. 1 shows cleaning positions 15 and 16 of the pipetting needle. In FIG. 1 arrows represent the sense of motion of the pipetting needle during the above-mentioned dispensing operations.
Drops can be dispensed in a number of ways. For instance, in inkjet printers a pressure pulse is generated within a liquid and this pulse propagates towards a nozzle which closes one end of a container containing the liquid to be dispensed. Due to the reduction of the cross-section at the transition from the interior of the container to the nozzle, a small liquid volume is strongly accelerated and this allows to release through the nozzle one drop of liquid from the container. The size of a drop generated by the inkjet principle lies in a range going from 5 to 500 picoliter and depends upon properties of the liquid and the size of the nozzle. Drops generated only by the weight of the drop to be dispensed are much larger. When a pipetting needle having a cross-section with an external diameter of 10 micrometers is used for dispensing drops only by means of gravitational force (i.e. the weight of the drops) the size of each drop would be of 30 nanoliters if the liquid dispensed is an aqueous solution.
When dispensing drops by the inkjet principle, a very strong acceleration of the liquid volume in the nozzle is necessary (accelerations of up to 105 g). The energy required for releasing a 500 picoliter drop is of about 10−8 Joule.
It is believed that a nozzle of the type used in inkjet printers cannot be a part of a pipetting needle of a device for analyzing samples for medical analysis, because the structure of the pipetting needle should allow the sufficient cleaning, but the presence of a nozzle in the structure of the pipetting needle would render this difficult.
Other features that a pipetting needle may fulfill are: (i) being suitable for piercing a closure of a liquid container, and (ii) having an elongated shape that is sufficiently long to penetrate to a predetermined level in a liquid container.